On the Relaxation Mechanisms of 6-Azauracil

Abstract

The nonadiabatic photochemistry of 6-azauracil has been studied by means of the CASPT2//CASSCF protocol and double-ζ plus polarization ANO basis sets. Minimum energy states, transition states, minimum energy paths, and surface intersections have been computed in order to obtain an accurate description of several potential energy hypersurfaces. It is concluded that, after absorption of ultraviolet radiation (248 nm), two main relaxation mechanisms may occur, via which the lowest (3)(ππ*) state can be populated. The first one takes place via a conical intersection involving the bright (1)(ππ*) and the lowest (1)(nπ*) states, ((1)ππ*/(1)nπ*)(CI), from which a low-energy singlet-triplet crossing, ((1)nπ*/(3)ππ*)(STC), connecting the (1)(nπ*) state to the lowest (3)(ππ*) triplet state is accessible. The second mechanism arises via a singlet-triplet crossing, ((1)ππ*/(3)nπ*)(STC), leading to a conical intersection in the triplet manifold, ((3)nπ*/(3)ππ*)(CI), evolving to the lowest (3)(ππ*) state. Further radiationless decay to the ground state is possible through a (gs/(3)ππ*)(STC).

Photochemistry of Nucleic Acid Bases and Their Thio- and Aza-Analogues in Solution

“…The increase in the Scheme 6 Proposed relaxation mechanisms of 6-azauracil depicting the multiple pathways that lead to the T 1 (ππ*) state. Reprinted with permission from [25]. Copyright 2011 American Chemical Society internal conversion rate from the S 1 (nπ*) state to the ground state caused by intramolecular vibrational energy relaxation in 6-azauridine vs 6-azauracil [15] is in agreement with the relaxation model proposed by West et al [193], but in contrast to that by Hare et al [140,159].…”

“…The high triplet yield provides strong evidence that intersystem crossing to the triplet state is the primary Excited at 308 nm i The intersystem crossing lifetime was estimated to be <1 ps and a self-quenching rate constant of (6.6 AE 0.8) Â 10 8 M À1 s À1 was measured relaxation pathway in 6-azauracil and 6-azauridine, whereas internal conversion to the ground state, from either the S 1 (nπ*) or S 2 (ππ*) states, is inefficient. Calculations performed by the groups of Marian [20] and Borin [25] at the DFT/ MRCI/COSMO/cc-pVDZ and CASPT2//CASSCF/ANO-L levels of theory, respectively, support the above mechanism. Etinski and Marian [20] concluded that the S 1 (nπ*) state acts as a doorway state in the population of the T 1 (ππ*) state, where the S 2 (ππ*) state population is transferred nearly quantitatively to the S 1 (nπ*) state.…”

“…Hence, according to these authors, the accompanying loss in planarity of the 6-azauracil ring upon relaxation to the S 1 (nπ*) state does not lead to a decrease in the spin-orbit coupling interaction in the 6-azauracil and 6-azauridine, as was proposed in the case of the uracil derivatives by Hare et al [140,159]. Gobbo et al [25] found an S 1 (nπ*)/T 1 (ππ*) crossing near the S 1 (nπ*) state minimum with a spin-orbit coupling of 64.7 cm À1 . However, the S 1 (nπ*)/T 1 (ππ*) spin-orbit coupling at the position of the S 1 (nπ*) state minimum was not reported.…”

“…The authors estimated that the T 1 (ππ*) state is populated within 30 ps in acetonitrile. However, in contrast to Kobayashi et al [14,15] and to Gobbo et al [25], Etinski and Marian [20] proposed that intersystem crossing from the S 2 (ππ*) state to the T 2 (nπ*) state is not competitive with internal conversion from the S 2 (ππ*) state to the S 1 (nπ*) state.…”

“…Gobbo et al [25] ascribed the excitation-wavelength dependence in 6-azauracil to the accessibility of an additional relaxation pathway when excitation is performed at 248 nm vs 308 nm. Scheme 6 presents the most relevant relaxation pathways computed by these authors.…”